22 



B. Rumberg, P. Schmidt-Mende , J. 7/eikard and H.T. Witt 



1 o . Reaction scheme 



The results of this chapter are summarized in fig. 1o (left). In 

 chlorella and fresh chloroplasts Chl-aj is reduced by electrons 

 originating from water with the help or hVjj-light . After sup- 

 pressing water-oxidation (by aging, extraction of plastoquinone , 

 addition of CMU) , Chl-at can be reduced by backflow of electrons 

 from Z~ or by added reduced FMS or reduced DPIP. In these circum- 

 stances one light reaction cycle (1) of the overall electron 

 transport system of photosynthesis has been completely isolated 

 (1o)(1'1 )(1) . This reaction cycle must be responsible for the sy- 

 stem that operated when Vernon, Zaugg and Kamen (12) used aged 

 chloroplasts with reduced substances which are capable of reduc- 

 ing TFN in the light. 



II. FHOTQREDUCTION OF X AND PLASTOQUINONE RESPECTIVELY 



1 . Analysis of X 



Out of the overall difference spectrum (under "normal" conditions) 

 we isolated one part: the Chl-aj-spectrum with peaks at 43o and 

 7o3 mu. 



The other part with peaks around ^75 mu and 5^5 mu has been 

 investigated already in detail (15)(1o). The results are summa- 

 rized in fig.lo (right). Instead of X we introduced already Q in 

 fig.lo (see below). Part of the changes are caused by a photore- 

 duction of a substance X (redox potential — Volt) . The elec- 

 tron donor is Y (redox potential:?' +o,8 Volt). Y oxidizes water. 

 The natural electron acceptor must be according to the result of 

 the last chapter - finally Chl-at . Added oxidized substances as 

 surnamed under oSjj act as artificial electron acceptors. That 

 the cycle is sensitized by a chlorophyll-a (Chl-ajj) follows 

 from the action spectra (11). In the following we will give fur- 

 ther evidence for the probability of this scheme. 



2. Separation of the difference spectrum of X 



To separate the reaction cycle of X from the natural electron 

 acceptor (Chl-a-j-), one has firstly to trap the electrons of X by 

 addition of ox.Sjj (for instance ox. DPIP). To suppress any reac- 

 tion of Ghl-a-p (backflow of electrons from Z~ to Chl-a-j-), one 

 has secondly •fco oxidize permanently Chl-aj by further addition 

 of ferricyanide . Ferricyanide keeps also DPIP permanently oxidiz- 

 ed. Fig. 11 shows that with addition of DPIP (1o"^ M/1) and ferri- 

 cyanide (1o~5 M/1) the difference spectrum of Chl-aj vanishes 

 and that of X is separated (1o)(2o)(1). In these circumstances a 

 second light reaction cycle (II) of the overall electron-trans- 

 port system has been completely isolated. This reaction cycle 

 must be responsible for the system which was described by Losada 

 et al (11) (oxygen-producing chloroplasts with ferricyanide and 

 DPIP) . 



3 . Relation of X and Plastoquinone-content 



a) Bishop (lA-) has shown that plastoquinone is somewhere in- 

 volved in the electron transport system of photosynthesis. The 



